From: Marc Postman (postman@stsci.edu)
Date: Thu Jan 05 2006 - 08:57:32 PST
Saul,
Here are two examples of relative surface densities of galaxies as a
function of their color, cluster membership, and elliptical morphology.
While it is true that the total number of cluster ellipticals is often
not that much greater than the total number of field ellipticals for a
given ACS pointing, the cluster ellipticals are all at the SAME
redshift whereas the field ellipticals span a large redshift range.
Therefore if one wants to probe a given redshift, using a cluster is by
far the better way to go. I think we can quantify this as well in a
separate plot. For example, in MS1054 there are 61 field ellipticals
and 81 cluster ellipticals with i <= 23.5 in the same ACS pointing. In
RXJ1252-29, there are 53 field ellipticals and 53 cluster ellipticals
with z <= 24.5 in the same ACS mosaic. But the 53 cluster ellipticals
are obviously all at z=1.24 (modulo errors in photo-z) whereas the
field ellipticals span a much broader range and largely sample lower
redshifts.
I don't think it will be easy to generate similar figs for our other
clusters in time for the proposal deadline, however. A lot of work is
needed to generate photo-z info and morphological info.
Marc
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Marc Postman Email: postman@stsci.edu
STScI - CMO Phone: +1-410-338-4340
3700 San Martin Drive Fax: +1-410-338-4424
Baltimore, MD 21218 U.S.A. http://www.stsci.edu/~postman
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